Chapter 2 Introduction to flow measurement
- 2. “ ▪ • Flow meters ▪ • Pressure transmitters ▪ • Temperature sensors ▪ • Level transmitters 2
- 4. Topics ▪ Introduction to Flow Measurement ▪ Flow Measuring Device: 1. Differential Pressure Meter - Orifice, Venturi,Nozzle 2. Electromagnetic Flowmeter 3. Vortex Flowmeter 4. Turbine Flowmeter 5. Ultrasonic Flowmeter - Transit Time Model, Doppler Ultrasonic Flowmeters 4
- 5. Topic outcomes ▪ Describe the flow measurement. ▪ Explain the basic operation, list the advantages and disadvantages of each flow measurement device. 5
- 6. What is “flowrate”? ▪ Amount of material passing one point for certain time. ▪ Give one simple method to measure… …water flowrate from tap water. 6
- 7. Why flow rate is important variable? ▪ Most process involves moving material from one part of the plant to another . ▪ Flow of 1. Material - Gas, liquid, solid particle 2. Energy 7
- 8. Common terms ▪ Velocity = measure the speed and direction the object, the rate of flow of fluid particles in a pipe. The units; feet per second (fps), meters per second (mps), and etc. ▪ Laminar flow = when its average velocity is comparatively low and the fluid particles tend to move smoothly in layers. ▪ Turbulent flow = when the flow velocity is high and the particles no longer flow smoothly in layers and turbulence or a rolling effect occurs. ▪ Viscosity = property of a gas or liquid that is a measure of its resistance to motion or flow. 8
- 9. Differential Pressure Meter • Sensors that rely on the pressure drop or head occurring as a fluid flows by a resistance or obstruction. • Velocity is increased after passing the obstruction while pressure is decreased. q= Flow rate K = constant ∆P= Pressure difference • Types of differential pressure meter; orifice meter, venturi meter, and flow nozzle. p K q = 9
- 11. ▪ An orifice plate is a restriction with an opening smaller than the pipe diameter which is inserted in the pipe perpendicular to the flow stream. ▪ Because of the smaller area the fluid velocity increases, causing a corresponding decrease in pressure. ▪ Typical orifice meter has a concentric, sharp edged opening Orifice plate Orifice Plate 11
- 12. Orifice Plate 12
- 13. ▪ Advantages Most commonly used flow sensor- inexpensive, easy to install, no moving part, simple configurations, required virtually no maintenance ▪ Disadvantages Creates low recoverable pressure (may recover only 50% of pressure drop) due to turbulence around the plate – high energy consumption. Orifice plate 13
- 14. Example Orifice calculation ▪ Q is the volumetric flowrate, Ao is the area of the orifice, ρ (kg/m3), is the liquid density, ΔP (Pa) is the pressure drop, P1-P2 ▪ From the Bernoulli equation, V (m/s) is the velocity 𝑄 = 𝐶𝑓𝐴𝑜 2∆𝑃 𝜌 ∆𝑃 = 𝑃1 − 𝑃2 = 1 2 𝜌𝑉2 2 − 1 2 𝜌𝑉1 2 14
- 15. Example Orifice calculation ▪ The flow coefficient Cf is found from experiments and is tabulated in reference books; it ranges from 0.6 to 0.9 for most orifices. ▪ The mass flowrate can be found by multiplying Q with the fluid density, 𝑚 = 𝑄𝜌 15
- 16. Derivation of orifice equation ▪ Bernoulli equation 𝑃1 + 1 2 𝜌𝑉1 2 = 𝑃2 + 1 2 𝜌𝑉2 2 ▪ Continuity equation 𝑞 = 𝑉1𝐴1 = 𝑉2𝐴2 ▪ Combine both equations 𝑃1 − 𝑃2 = 1 2 𝜌𝑉2 2 − 1 2 𝜌𝑉1 2 ▪ Rearrange the equations ∆𝑃 = 1 2 𝜌 𝑞 𝐴2 2 − 1 2 𝜌 𝑞 𝐴1 2 ∆𝑃 = 1 2 𝜌𝑞2 1 𝐴2 2 − 1 𝐴1 2 ∆𝑃 = 1 2 𝜌𝑞2 1 𝐴2 2 1 − 𝐴2 𝐴1 2 16
- 17. Derivation of orifice equation ▪ Determine q 𝑞 = 2∆𝑃 𝜌 𝐴2 1 − 𝐴2 𝐴1 2 17
- 18. • It consists of a convergent cone section, cylindrical bore, and a divergent cone section. • The cylindrical bore restricts the fluid flow resulting in a pressure drop. • The change in cross sectional area causes a pressure change. • Can be used on slurries and dirty fluids, high installation cost. Venturi meter 18
- 19. Venturi meter ▪ More expensive than orifice meter but produce substantially higher recoverable pressure drops recovers up to 85% of pressure drop), due to the continuous geometry of the contraction, throat, and divergent sections. 19
- 20. Example Venturi meter calculation 20 Q is the volumetric flowrate, A and A1 are the areas in the venturi, ρ (kg/m3), is the liquid density, P and P1 are the pressures in the venture, v (m/s) is the velocity Therefore; A A1 𝑃 − 𝑃1 = 1 2 𝜌𝑉1 2 − 1 2 𝜌𝑉2 𝑄 = 𝑣𝐴 = 𝑣1𝐴1 𝑄 = 𝐴 2 𝜌 𝑃1 − 𝑃 1 − 𝐴 𝐴1 2
- 21. ▪ Advantages Widely used for high flow rates. They can handle 25- 50% more flow rate than an orifice. ▪ Disadvantages The pipe design need not be straight, insensitive to the changes in the velocity of flow Venturi meter 21
- 22. ▪ Can handle flowrates that are larger than the capabilities of orifice plate with acceptable pressure loss ▪ Consist of an elliptical converging section and cylindrical throat section. ▪ Flow Nozzles have a smooth elliptical inlet leading to a throat section with a sharp outlet. This restriction in the fluid flow causes a pressure drop. Flow nozzle 22
- 23. Advantages ▪ Suitable for high-velocity, non-viscous, erosive flows. ▪ produce less differential pressure (low pressure loss) than venturi Disadvantages ▪ offers lower than venturi installation costs but higher than orifice Flow nozzle 23
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- 25. ▪ This instrument operates based on Faraday’s Law. ▪ It is ideal for liquids that conduct electricity. ▪ A charged particle moving through the magnetic field produces a voltage proportional to the velocity of the particle. ▪ The magnetic field is developed by electric coils. Electromagnetic flowmeter 25
- 26. Electromagnetic flowmeter ▪ This conductor in the magnetic field will generate an electric voltage that is proportional to its average velocity. ▪ The pipe section in which measurement is made must be insulated or nonconductor itself, so that the generated voltage not dissipated through the pipeline ▪ A pair of electrodes is installed across the pipe wall to detect the induced voltage. 26
- 27. Advantages: ▪ Minimum pressure drop ▪ can be used in hazardous environments or measure corrosive or slurry fluid flow ▪ low maintenance cost because of no moving parts ▪ high linearity between the output and flow rate ▪ more accurate than differential pressure meter (orifice, venturi and flow nozzle) since this flow meter does not introduce any pressure drop Electromagnetic flowmeter 27
- 28. Disadvantages/Limitations ▪ material must be liquid that conduct electricity ▪ requires electrical conductivity of fluid higher than 3 µS/cm in most cases, (particle caused bias in reading). ▪ expensive Electromagnetic flowmeter 28
- 29. ▪ Vortex meter operate on the principle that when a non- streamlined object (barrier/bluff body) is placed in the middle of flow stream ▪ A series of vortices are shed alternately downstream of the object ▪ The frequency of vortex shedding down each side of the bluff body is directly proportional to mean flow velocity and therefore to volume flow. ▪ A device that counts the vortices passing per second will also measure the flowrate. Vortex flowmeter 29
- 30. Advantages ▪ Low cost installation-do not required impulse tubing and valve manifold. ▪ Universally suitable for measuring liquids, gases and steam ▪ Largely unaffected by changes in pressure, temperature and viscosity ▪ No moving parts ▪ Marginal pressure loss ▪ Easy to install and commission Vortex flowmeter 30
- 31. Vortex flowmeter Disadvantages ▪ Vortexes are inhibited in viscous fluid at low flow rate. ▪ At high fluid velocity the obstructions may introduce excessive pressure drop-limited to higher flow rate. ▪ Vaporization of liquid may damage the vortex detector-care must be taken. 31
- 32. ▪ Turbine meters have a spinning rotor with blades that is mounted on bearings in a housing on the central longitunidal axis of the pipeline. ▪ The rotor spins as water or other fluid passes over it. ▪ Magnets are embedded in the rotor housing and a pickup coil, isolated from the fluid is placed outside the rotor blades. ▪ The rotating magnets induce a voltage pulse in the coil each time they pass the coils. Blade movement is often detected magnetically. ▪ The pulse frequency is proportional to the velocity of the fluid. When the fluid moves faster, more pulses are generated Turbine 32
- 33. Turbine ▪ Turbin meter excel at measuring steady, low flows of liquid and gas. Widely used in utility applications to measure the amount of water used in commercial and industrial buildings. ▪ Have excellent accuracy. ▪ The size (diameter) same as the pipe in which they are fitted, and pressure loss is quite low. ▪ Disadvantages: limited to clean fluid, require maintenance at their bearing from time to time, expensive. ▪ Acceleration of fluid beyond the normal at upstream cause excessive rotational speed and have serious consequences on the life of the meter. 33
- 34. ▪ Advantages Suitable for high pressure measurement, high accuracy and repeatability. Measurement of non- conductive liquids ▪ Disadvantages Can only measure clean fluids, and filters are normally needed when installing the sensor. Not effective with swirling fluids. Not applicable to higher viscosity fluids. Turbine flowmeter 34
- 35. ▪ Ultrasonic flowmeters can be categorized into two types based on the installation method: clamped-on and inline. ▪ The clamped-on type is located outside of the pipe and there are no wetted parts. It can easily be installed on existing piping systems without worrying about corrosion problems. ▪ Clamped-on designs also increase the portability of the flowmeter. ▪ The inline type, on the other hand, requires fitting flanges or wafers for installation. However, it usually offers better accuracy and its calibration procedures are more straightforward. Ultrasonic flowmeter 35
- 36. • A pair of transducers is placed on the pipe wall, one on the upstream (transmitter) and the other on the downstream (receiver). • It sends pulses of ultrasonic energy diagonally across the pipe. • Flowrate is measured based on the time for the sound to travel between a transmitter and a receiver • The time for acoustic waves to travel from the upstream transducer to the downstream transducer is shorter than the time it requires for the same waves to travel from the downstream to the upstream. Time Transit Model - for clean fluids transmitter receiver 36
- 37. ▪ Use only one transducer for transmitting and receiving ultrasonic frequency ▪ Apply principle of frequency shift between the ultrasonic frequency source, receiver and fluid carrier due to the fluid velocity. ▪ Usually requires some particles or undissolved gasses (bubble) in the flow to reflect the signals. ▪ As the objects are moving, the reflected ultrasonic energy will have a different frequency. The amount of difference between the original and returned signals is proportional to the flow velocity. Doppler ultrasonic flowmeter - for dirty, slurry-type flow 37
- 38. ▪ Advantages No moving parts, low maintenance and high accuracy. No obstruction in the flow path, no pressure drop. Can be used to measure corrosive or slurry fluid flow. ▪ Disadvantages Higher upfront cost, cannot be used for heavily contaminated liquids or slurry. Turbulence or even the swirling of the process fluid can affect the ultrasonic signals. Ultrasonic flowmeter 38
- 39. “ There are key questions you can ask yourself when trying to determine which flowmeter is the best choice for you. The purpose of the measurement and the physical characteristics of the fluid being measured are the two main considerations. How to choose the right flowmeter? 39
- 40. Some factors to consider are ▪ Type of the fluid being measured (air, water, gas, oil, slurry, etc.) ▪ Fluid properties (clean, dirty, viscous, slurry, corrosive, conductive etc.) ▪ Measurement device accuracy, turndown and repeatability ▪ Total cost involve (equipment cost, total cost installation, maintained cost, and operating cost)- cheap or expensive? ▪ Type of construction materials (stainless steel, fiberglass, etc.) ▪ Pressure loss (high, low, intermediate?) 40
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- 42. Vortex Flowmeter Orifice plate Venturi meter Turbine flowmeter Electromagnetic flowmeter Flow Nozzle Time Transit Model Doppler Electromagnetic flowmeter 42
- 43. Thanks! 43